This invention relates to constant voltage output circuits, and more specifically relates to a novel temperature-stable constant output voltage circuit which can be implemented with MOSFET transistors on the same monolithic chip containing other circuits which are to be connected to the constant reference voltage source.
The use of MOSFET devices to create complex circuits on a single semiconductor chip is well known. A typical circuit of this type is an analog-to-digital converter, or A/D converter, disclosed in Electronics Letters Sept. 16, 1976, Volume 12, No. 19, pages 491 to 493, entitled CHARGE-TRANSFER ANALOGUE-TO-DIGITAL CONVERTER, by W. J. Butler and C. W. Eichelberger. Most A/D converters of the type disclosed in the above article require a stable reference voltage in order to make accurate measurements and to provide an output reading in absolute units. The performance of the converter is directly determined by the stability of the reference voltage source and, as a result, the generation of a stable, termperature-insensitive reference voltage plays an important part in the proper functioning of the circuit.
At the present time, reference voltage generators normally employ bipolar devices and, therefore, cannot be processed or placed on the same monolithic chip with an A/D converter which is implemented by MOSFET devices. Therefore, it has been necessary, when an A/D converter or other similar type device is implemented by MOSFET techniques, to have a stable reference voltage supplied by some external circuit off the chip. This is also often true when the implementation of the A/D circuit is with bipolar devices since the processing required for a temperature-stable bipolar reference such as a zener diode, is not always compatible with the A/D circuit process requirements. Thus, at the present time there is no monolithic A/D converter which is complete in itself and all known devices require an external reference voltage chip or other circuit, the cost of which may be comparable to that of the cost of the A/D converter.